Navigation dropdown fix for mobile browsers

This fix is solving the problem with inaccessible (untouchable) menu items from navigation dropdowns in mobile browsers.

Changes:
before: ("ontouchstart"in document.documentElement&&e('<div class="dropdown-backdrop"/>')
after: ("ontouchstart"in document.documentElement&&(!s.closest('.nav').length)&&e('<div class="dropdown-backdrop"/>')

Same bug reports for Twitter Bootstrap 2.3.2:
https://github.com/twbs/bootstrap/issues/4550
https://github.com/twbs/bootstrap/pull/6488

Same fix in Twitter Bootstrap v3.0:
if (!isActive) {
      if ('ontouchstart' in document.documentElement && !$parent.closest('.navbar-nav').length) {
        // if mobile we we use a backdrop because click events don't delegate
        $('<div class="dropdown-backdrop"/>').insertAfter($(this)).on('click', clearMenus)
      }